Background: Streptococcus equi subspecies zooepidemicus is a highly diverse opportunistic pathogen of horses, associated with respiratory disease and endometritis. Objectives: To characterise S. zooepidemicus isolates recovered from young ponies during a natural episode of respiratory disease, and to determine if vaccination with Strangvac, a vaccine against Streptococcus equi subspecies equi, conferred cross-protection. Study design: Retrospective analysis of a double-blinded placebo-controlled study involving 32 ponies (16 vaccinates and 16 placebo controls) that experienced a natural episode of respiratory disease around the time of second vaccination. Methods: Ponies were monitored daily for clinical signs of respiratory infection. Nasopharyngeal swabs were taken from affected animals to identify equine pathogens by qPCR and culture. The genomes of 23 S. zooepidemicus isolates were sequenced. Clinical and serological differences between vaccine and placebo groups were determined. Results: The clinical samples were positive for equine herpes virus-4 or S. zooepidemicus (1/15 (7%) and 13/15 (87%), respectively). Seven different S. zooepidemicus sequence types, which encoded between four and seven of the eight antigens in Strangvac with ≥70% amino acid identity, were recovered from affected horses. The most conserved antigens were CNE, EAG, Eq5 and IdeE. The number of days that ponies had a cough were significantly associated ( p=0.005) with the vaccination status, with fewer days in the vaccinated compared to co-mingled control ponies (1.8±1.9 days vs. 3.4±3.5 days). A higher antibody titre to IdeE immediately prior to second vaccination correlated with a lower cumulative coughing score ( p=0.03). A reduced number of days with abnormal temperature correlated with increased antibody levels against Eq85 ( p=0.023-0.028). Main limitations: Small group size. Conclusions: This study provides evidence in support of a cross-protective effect of Strangvac for the reduction of clinical signs associated with natural infection with S. zooepidemicus, which correlated with the presence of higher antibody titres to IdeE and Eq85.

Background: Strangles, caused by Streptococcus equi, is an endemic disease of horses throughout the world. A vaccine against strangles, Strangvac, is available in Europe, but there are no published serological data following the revaccination of Sport horses. Objectives: To measure the antibody response of Sport horses to the components of Strangvac before and after third vaccination. Study Design: Longitudinal cohort study. Methods: Adult Sport horses were administered their third vaccination with Strangvac at 5 months (group 1, n=20) or 12 months (group 2, n=15) after second vaccination. Horses were monitored closely for adverse signs. Blood samples were collected from both groups immediately before and 14 days after third vaccination to quantify antibody responses by ELISA. Results: Transient signs of mild injection site swelling in 8.5 % horses for 2 days, and a temperature rise of ~0.8 oC in most horses for 1 day, occurred post-vaccination. Health status returned to normal within 48 hours. The antibody titres to the vaccine components were 3.34±0.31 and 3.14±0.29 before third vaccination and increased significantly to 3.90±0.29 and 3.90±0.17 after third vaccination in groups 1 and 2, respectively (P<0.0001 for both groups). Main limitations: Data and samples from a matched unvaccinated control group were unavailable. Conclusions: Strangvac was safe and a revaccination program after 5 or 12 months was equally efficient for the vaccination of Sport horses.

Background: Strangles, caused by Streptococcus equi subspecies equi ( S. equi), is a prevalent infectious disease of horses. This is the first report on protection conferred by a new vaccine, Strangvac, against natural infection. Objectives: To measure the effects of Strangvac vaccination during an outbreak of strangles at a Swedish farm. Study Design: Longitudinal cohort study. Methods: Healthy horses ( n=17) were vaccinated on day 0, which occurred 23 days after strangles was first confirmed in three unvaccinated horses on the same farm. Blood serum samples were collected on day 0, day 28 and day 489. A combined iELISA was used to measure total antibody titres towards the components of the vaccine. Specific antibody levels in blood sera to S. equi were also quantified using the antigen A/C iELISAs, which can differentiate infected from vaccinated animals. Clinical signs were monitored. Results: All vaccinated horses had increased total antibody titres to CCE, Eq85 and/or IdeE from day 0 (2.50±0.28) to day 28 (3.63±0.31; p<0.0001). Seropositivity in the antigen A/C iELISA was noted in 8/17 healthy horses on day 0 (prior to vaccination), in 9/16 vaccinates on day 28, and in 3/3 unvaccinated clinical cases on day 28. None of the vaccinated horses developed strangles, but 5/7 of the original vaccinated horses tested seropositive in the A/C iELISA on day 489, which was 377 days after their third vaccination. Main limitations: Aside from the three unvaccinated clinical cases, no unvaccinated control group was available. Conclusions: Despite adherence to recommended biosecurity measures, serological evidence of exposure to S. equi was demonstrated in half of the vaccinated horses. Notably, all horses vaccinated during the outbreak responded to the vaccine components and remained healthy, supporting the protective effects of vaccination of healthy horses with Strangvac in outbreak situations.